(1) Field of the Invention
The present invention relates to cellular radio communication technology that adopts orthogonal frequency division multiplex (OFDM) in radio communication.
(2) Description of the Related Art
Research and development is underway on radio communication systems that adopt OFDM (Orthogonal Frequency Division Multiplex). OFDM produces data to be transmitted in a frequency domain, converts it into a signal of a time domain by IFFT (Inverse Fast Fourier Transform), and transmits it as a radio signal. A receiving side converts the signal of the time domain into a signal of the frequency domain by FFT (Fast Fourier Transform) to extract its original information.
An OFDM cellular radio communication system generally includes plural base station apparatuses and plural terminals, as shown in FIG. 1. A base station apparatus 101 is connected to a network 102 over a wire line. Terminals 103, 104, 105, and 106 are wirelessly connected with the base station apparatus 101 to be communicatable with the network 102. Effective communication with a base station require radio channel conditions of a given level or higher, and are generally governed by a distance from the base station. A range communicatable with a certain base station is referred to as a cell, which has a circular shape like 107 when no shielding matter exists. Terminals perform communication with a base station having the best radio channel condition. Therefore, in the example of FIG. 1, the terminals 103, 104, 105, and 106 that exist in a cell within the base station 101 communicate with the base station 101 as a communication destination.
When the number of communication terminals per base station is many like cellular radio, the base station communicates with plural terminals at the same time using directional beams 201, 202, and 203 having different directions, as shown in FIG. 2. In this case, one cell is logically split by the number of directional beams; the logically split unit is referred to as a sector.
FIG. 2 shows an example of CDMA (Code Division Multiple Access) 2000 1xEV-DO (Evolution Data Only) system. The number of sectors is 3, and the terminals 103 and 106 use a beam 201 and the terminals 104 and 105 use beams 202 and 203, to communicate with the base station 101. Hereinafter, sectors corresponding to the beams 201, 202, and 203 are defined as sectors 1, 2, and 3.
In FIG. 1, when a terminal in a cell boundary receives data transmission from a base station, since interference power (power originating in a base station other than a communication destination) is stronger than signal power of the base station of a communication destination, channel quality deteriorates. This is also true for terminals in sector boundaries in FIG. 2. As means for reducing the influence, for example, as shown in FIG. 3, frequency hopping patterns of OFDM can be used. FIG. 3 shows different patterns for different sectors. To perform communication with a given user, the sector 1 uses time and frequency such as a pattern 301, and the sector 2 uses a pattern 302 likewise. In an identical sector, patterns with offset of a frequency direction appended are used to avoid the overlap of time and frequency resources of individual users. Use of such patterns 301 and 302 reduces the rate of the overlapping of time and frequencies with users of other sectors such as 303. Since the terminals demodulate corresponding frequencies every hour, the hopping helps to suppress interference power. As shown in FIG. 4, by allocating mutually different hopping patterns 401, 402, and 403 to the sectors 1, 2, and 3, each sector can communicate with different terminals at the same time with interference suppressed.
The standardization group IEEE802.20 proposes a radio system based on OFDM, and defines an interference suppression method by the above-described hopping patterns in Section 9.3 of IEEE C802.20-06/04.
The standardization group 3GPP proposes a radio system based on OFDM as LTE (Long Term Evolution), and defines an interference suppression method by the above-described hopping patterns in Section 7.1.2.6 of 3GPP TR 25.814 V7.0.0 (2006-06).
Furthermore, the standardization group 3GPP2 proposes a radio system based on OFDM as LBC (Loosely Backwards Compatible), and defines an interference suppression method by the above-described hopping patterns in Section 1.1 of 33GPP2 C30-20060731-040R4.
In the related art, during communication with different terminals, channel quality greatly changes depending on terminal positions. For example, as shown in FIG. 5, a terminal existing in the front direction of the beam 201 such as the terminal 106 can have high channel quality, while a terminal off the direction of the beam 201 such as the terminal 103 can obtain only low channel quality. Since channel quality information corresponds to data rates, in such a case, the terminal 103 may not obtain a desired data rate.
As one of measures against such a problem, a method of plural sectors operating as substantially one sector by using a same hopping pattern is proposed in IEEE802.20. An example of matching the hopping pattern 402 of the sector 2 to the hopping pattern 401 of the sector 1 is shown in FIG. 6. Since the terminal 103 enables synthesis in signal level by receiving the beams 201 and 202 at the same time, channel quality increases in comparison with reception of the beam 201 alone.
On the other hand, as a method for backing up terminals in cell boundaries, for example, as disclosed in Japanese Patent Application Laid-Open Publication No. H05-110499, for communication with terminals near a base station, hopping patterns permitting concurrent use of other cells and frequencies are used, while for communication with terminals in cell boundaries, patterns not permitting concurrent use is used. This method enables an improvement in channel quality of terminals in cell boundaries while suppressing deterioration of the number of frequency repetitions by switching of hopping patterns.